This invention relates to development systems for electrostatic processors and, more particularly, to housings for those systems.
In a conventional electrostatic printing process of the type described in Carlson's U.S. Pat. No. 2,297,691 on "Electrophotography", a uniformly charged photoconductor is selectively discharged in an image configuration to provide a latent electrostatic image which is then developed through the application of a finely divided marking material, called "toner". As is known, that process has enjoyed outstanding commercial success, especially in plain paper copiers and duplicators. Nevertheless, substantial effort and expense are still being devoted to the perfection of the process, including the development step.
The modern practice is to carry out the development step on the fly -- viz., as the photoconductor moves through a development zone. There are various ways to accomplish that, but magnetic brush development has recently come into favor because it is capable of providing superior solid area coverage while satisfying the normal requirement for minimal background development.
Characteristically, a magnetic brush development system includes one or more development rolls which are rotated in a shaped magnetic field to brush the image bearing photoconductor with a multi-component developer containing toner particles and larger, ferromagnetic "carrier" particles. The toner and carrier (or, sometimes, carrier coating) components of the developer are formed from materials which are separated from one another in the triboelectric series, with the result that electrical charges of opposite polarities tend to be imparted to the two different types of particles by a triboelectric charging process. Moreover, consideration is given to the triboelectric ranking of the materials selected so that there is a tendency for the toner particles to acquire a charge having a polarity opposing the polarity of the latent image.
In operation, developer is magnetically entrained on the development roll or rolls. The magnetic field is shaped so that the developer carried by those rolls tends to collimate as it is advanced through the development zone, thereby forming bristle-like stacks of developer which brush against the image bearing photoconductor. There are, of course, competing electrostatic forces acting on the toner particles, whereby those particles at least initially tend to be attracted to the carrier particles, but are subject to being electrostatically stripped therefrom when they are moved into the immediate proximity of or actual contact with the photoconductor.
As will be appreciated, the quality of the copies produced through the use of a magnetic brush development process are dependent on the spacing of the development roll or rolls from the photoconductor. Generally, an electrical field is established between those rolls and a supporting substrate for the photoconductor to suppress background development. Thus, any change in the development roll-to-photoconductor spacing not only alters the quantity of toner deposited on the image, but also varies the gradient of the field. It goes without saying that changes in either of those two parameters are necessarily accompanied by a corresponding change in the quality of the copies produced.
Unfortunately, it has not been practical with prior magnetic brush development systems to do anything more than approximate the optimum development roll-to-photoreceptor spacing. As a general rule, those systems can be serviced only after they have been moved, development rolls and all, away from the photoconductor. Accordingly, no particular effort has been made to maintain a constant development roll-to-photoreceptor spacing since precise repositioning of a development roll is a time consumming task at best.